Put cubed sweet potatoes in a steamer basket. Place the basket in a large pot of simmering water that is no closer than 2 inches from the bottom of steamer. Allow potatoes to steam for 20 minutes of until fork tender. Mash cooked potatoes and set aside.
In a large bowl, whisk together flour, baking powder, and salt and set aside.
In another bowl combine the sweet potatoes, milk, brown sugar, butter, and grated orange rind. Stir the sweet potato mixture into the flour mixture and thoroughly combine. Beat egg whites until stiff peaks form. Gradually fold egg whites into batter 1/3 at a time. The batter will be thick. Using a No. 20 disher (scoop), place 2 scoops of batter onto a preheated, oiled waffle iron, and cook until lightly browned, about 5 to 6 minutes.